1. Field of the Invention
The present invention relates to a method for heat treatment of a silicon substrate produced by the Czochralski method (CZ method), and an epitaxial wafer produced by using a substrate heat-treated by the method. In particular, it relates to a method for heat treatment for obtaining a semiconductor substrate imparted with internal gettering ability through a high temperature heat treatment in a short period of time.
2. Description of Related Art
Silicon crystals produced by the CZ method contain not a little oxygen, because a crucible made of quartz is used for the production. It has been known that this oxygen forms precipitates during the production of the crystals and the heat treatment in the device fabrication process after slicing and substrate processing of the crystals.
When the oxide precipitates are in the device active regions, they may be a cause of degradation of device yield. On the other hand, the oxide precipitates formed in the inside of substrates afford an effect for improving gettering ability for heavy metal contamination species. From these points of view, it is extremely important to control the amount of oxide precipitates in a silicon substrate (control of oxygen precipitation characteristic).
The control of this oxygen precipitation characteristic has conventionally been achieved by controlling the crystal oxygen concentration during the production of silicon crystals, or by subjecting the substrates to a heat treatment for a long period of time.
However, the former type control requires establishment of crystal production conditions for a desired oxygen concentration. Therefore, the production operation becomes complicated, and in addition the accuracy is bad. Further, even though a crystal have a constant oxygen concentration along the crystal growth direction, the oxygen precipitation characteristic along the growth axis direction is not necessarily uniform due to influence of the thermal history during the crystal production. To obtain uniform oxygen precipitation characteristic, heat treatment of long period of time is required, and hence the productivity becomes low.
To overcome the above situation, the method disclosed in Japanese Patent Application Laid-open (KOKAI) No. 3-77330 aims at eliminating the influence of the thermal history during the crystal growth, and obtaining a constant amount of precipitated oxygen without differently controlling the oxygen concentration for every substrate by forming an oxide film having a thickness of 100 angstrom (.ANG.) or more on a silicon substrate surface, then heat-treating the substrate at a temperature of from 1170.degree. C. to 1300.degree. C. in nitrogen gas atmosphere, cooling it at a cooling rate not less than 100.degree. C./minute, and then performing heat treatment for oxygen precipitation.
On the other hand, as for an epitaxial wafer manufactured by using a silicon substrate produced by the CZ method, when epitaxial growth is performed using a silicon substrate having a usual resistivity, potential oxygen precipitation nuclei formed during the substrate production (crystal production) are eliminated by the high temperature heat treatment in the epitaxial growth step, and thus the gettering effect for metal impurities may be lost. Insufficient gettering effect degrades the yield in the device processing step.
Further, the amount of precipitated oxygen may vary depending on the oxygen concentration of silicon substrates and thermal history during the crystal production. For example, when substrates having different initial oxygen concentrations are used, the amounts of precipitated oxygen vary, and hence different gettering abilities are resulted. In particular, a wafer of low oxygen concentration affords a low amount of precipitated oxygen. Therefore, to maintain stable gettering effect, it is preferred that wafers can have a constant amount of precipitated oxygen. This is not limited to epitaxial wafers, and this is also the case when usual silicon substrates are subjected to a heat treatment for obtaining the gettering effect.
Further, the device fabrication process comprises various kinds of heat treatments, and an optimum amount of precipitated oxygen exists for each process (heat treatment). Inter alia, when the substrates are used as a substrate for epitaxial growth, it is very important to control the amount of precipitated oxygen for the following reason. That is, when the oxygen precipitation is little, the gettering effect becomes insufficient, and when it is too much, the substrates may be, for example, warped. Therefore, it is necessary to obtain an optimum amount of precipitated oxygen (precipitation amount).
In order to control oxygen precipitation characteristic during various heat treatments, it has conventionally been required to perform various controls during the pulling of single crystals to control the oxygen concentration, or perform heat treatment for a long period of time as for substrates.
Further, in the method disclosed in the aforementioned Japanese Patent Application Laid-open (KOKAI) No. 3-77330, the atmospheric gas for the heat treatment is limited to nitrogen, and the heat treatment time is long, i.e., 15 minutes. Moreover, it requires formation of an oxide film for preventing out-diffusion of nitrogen impurities before the heat treatment, and therefore it utilizes an increased number of process steps. Furthermore, because it relies on cooling method by taking substrates out from the heat treatment furnace, it suffers a drawback of difficult handling.
Although the aforementioned method may uniformalize the precipitation amount varying depending on the location in crystals to some extent, it does not control the precipitation amount itself. Thus, it is important to control the amount of precipitated oxygen for the many heat treatment steps, and it is required to produce a silicon substrate containing precipitation nuclei in a desired amount, from which the precipitates are formed. Further, the precipitation amount must be made uniform as for substrates having different oxygen concentrations, and it is desired to develop a method capable of, for example, affording a sufficient precipitation amount even for a wafer of low oxygen concentration, and controlling the amount of precipitated oxygen (oxygen precipitation nuclei) itself.